1. Field of the Invention
The invention relates to electronic circuits, and more particularly, to a driver amplifier circuit.
2. Description of the Related Art
In a process that a signal is transmitted from a first unit to a second unit, if these two units have different DC voltage levels, a DC component is usually removed from the signal by means of AC-coupling (e.g., using a capacitor) before the signal enters the second unit. Then, the DC voltage level of the signal is reconstructed in the second unit.
FIG. 1 is a block diagram of a conventional earphone driver amplifier circuit. Assuming that an operational amplifier (Op-Amp) in FIG. 1 is supplied with 5V, then VDD=5V. The supply voltage VDD and a ground voltage are respectively supplied to a positive power supply terminal 111 and a negative power supply terminal 112 of an Op-Amp 110. The output signal Sout of the Op-Amp 110 contains a DC component equal to VDD/2. An output terminal of the Op-Amp 110 is connected in series with an AC-coupling capacitor 120 so as to remove the DC component contained in the audio signal Sout for reducing power consumption and avoiding damaging the speaker 130. In addition, it is necessary for low-frequency audio signals to pass through the capacitor 120 and the speaker 130 has a relatively low impedance, so the capacitor 120 has a capacitance within the range of several hundreds of microfarads. However, the capacitors in the several hundreds of microfarads have a higher hardware cost in addition to having a larger size and taking up more space.
U.S. Pat. No. 7,061,327 discloses another conventional method, causing an output audio signal to be biased at the ground voltage (i.e., the DC component equal to 0V) by providing a negative voltage (−VDD). Even though such design saves the trouble of configuring the AC-coupling capacitor 120, the voltage that the elements in the driver amplifier circuit may sustain is increased from VDD to (2×VDD) (meaning that the sizes of the elements need to be increased so as to increase the withstand voltages of the elements), otherwise the elements in the driver amplifier circuit will be damaged. Thus, there is a need to improve the prior art earphone driver amplifier circuits.